The present invention relates to an optical amplification apparatus for amplifying signal light by utilizing Raman amplification and a controlling method thereof. More particularly, present invention relates to an optical amplification apparatus for achieving the improvement of noise characteristics of the overall optical amplification apparatus by taking influences of noise light generated by Raman amplification into consideration, and a controlling method thereof.
Demands for information have been increased drastically in recent years with the progress of Internet technologies. A greater capacity and the formation of more flexible networks have been required in a trunk type optical transmission system in which an information capacity is integrated. A WDM optical transmission system for transmitting wavelength division multiplexed (WDM) signal light obtained by multiplexing a plurality of optical signals having different wavelengths is one of the most effective means that can cope with such a system demand. In a conventional WDM optical transmission system, an optical fiber amplifier using an optical fiber doped with a rare earth element such as erbium (Er) is utilized, as an optical repeater. By utilizing broadband characteristics of this optical fiber amplifier, one optical fiber can realizes WDM optical transmission for repeating and transmitting optical signals of a plurality of wavelengths.
To further increase the capacity and to extend the distance and the repeating interval in the WDM optical transmission system as described above, means for compensating for the degradation of S/N in the transmission system becomes necessary. For this purpose, it is effective to use means for supplying excitation light to a transmission path, to perform distributed Raman amplification of the transmission path by utilizing an amplification operation using the effect of stimulated Raman scattering, so that the repeating loss is equivalently reduced, in addition to an existing optical amplification repeating transmission system.
FIG. 8 is a structural diagram showing the outline of a WDM optical transmission system using distributed Raman amplification, which has been proposed heretofore.
In the WDM optical transmission system in FIG. 8, a transmission path 3 connects a transmission station (Tx) 1 and a reception station (Rx) 2, and a plurality of optical repeaters 4 are arranged on the transmission path 3 with predetermined intervals so that WDM signal light is transmitted and repeated from the transmission station 1 to the reception station 2. Each optical repeater 4 includes an optical amplification apparatus constituted by combining a DRA (Distributed Raman Amplifier) with an EDFA (Erbium-Doped Fiber Amplifier). In this DRA, excitation light for Raman amplification (hereinafter called xe2x80x9cRaman excitation lightxe2x80x9d) generated in an excitation light source is supplied through an optical coupler to the transmission path 3 connected to the transmission station side, and the WDM signal light propagated through the transmission path 3 is subjected to distributed Raman amplification. The WDM signal light subjected to the distributed Raman amplification is input to the EDFA, to be amplified to a necessary level, and is again output to the transmission path 3. With such a WDM optical transmission system, since the loss in the transmission path 3 in each repeating segment is decreased due to distributed Raman amplification, transmission characteristics of the WDM signal light can be improved.
Noise characteristics of the optical amplification apparatus constituted by combining the DRA with the EDFA and used for such a WDM optical transmission system are affected not only by the noise figure (NF) of the EDFA but also by noise light generated by the Raman amplification. The noise light resulting from Raman amplification is generated also when only Raman excitation light is incident to an amplification medium under a state where signal light is not input, and is generally called xe2x80x9cRaman scattering light due to pumping lightxe2x80x9d. Here, noise light generated in the DRA is called xe2x80x9cAmplified Spontaneous Raman Scattering (ASS) lightxe2x80x9d in contrast with Amplified Spontaneous Emission (ASE) light generated in the EDFA.
To improve the noise characteristics of the optical amplification apparatus and to further improve the transmission characteristics, it is necessary to reduce the noise figure of the overall optical amplification apparatus by taking the influences of ASS light into consideration. To improve the noise characteristics of the optical amplification apparatus, technologies for reducing independently the noise figure of the EDFA have been studied in the past, but specific considerations taking the influences of ASS into account have not been made.
The present invention has been made in view of the problems described above, and it is an object of the present invention to provide an optical amplification apparatus for achieving the improvement of noise characteristics by controlling an amplification operation by assuming a noise figure of an overall optical amplification apparatus while taking influences of noise light resulting from Raman amplification into account, and a controlling method of such an optical amplification apparatus.
To accomplish the object described above, an optical amplification apparatus utilizing Raman amplification according to the present invention comprises: first optical amplifying means for Raman amplifying signal light propagated through a Raman amplification medium by supplying excitation light to the Raman amplification medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, wherein the optical amplification apparatus further comprises: target value setting means for setting a target value for minimizing a noise figure of the overall optical amplification apparatus as to input light power of said second amplifying means; and excitation light controlling means for controlling an excitation light supply condition of the first optical amplifying means in accordance with the target value set by the target value setting means.
According to this construction, input light power of the second optical amplifying means to which Raman amplified signal light is input, is taken into specific consideration, and its target value is set by the target value setting means. The input light power target value of the second optical amplifying means minimizes the noise figure of the overall optical amplification apparatus constituted by combining the first and second amplifying means. As the excitation light supply condition of the first optical amplifying means is adjusted by the excitation light controlling means in accordance with the set target value, actual input light power of the second optical amplifying means is so controlled as to coincide with the target value. Consequently, the noise characteristics of the overall optical amplification apparatus, that takes the influences of noise light due to Raman amplification into consideration, can be optimized by the control inside its own apparatus, and an optical amplification apparatus having excellent noise characteristics can be realized.
As one aspect of the optical amplification apparatus described above, the target value setting means may include an excitation light power detecting section for detecting excitation light power supplied to the Raman amplification medium, and a computing section for computing noise light power by the first optical amplifying means in accordance with a detection result of the excitation light power detecting section, and setting an input light power target value of the second optical amplifying means for minimizing the noise figure of the overall optical amplification apparatus on the basis of the computed noise light power and on the basis of noise characteristics of the second optical amplifying means.
According to this aspect, in the computing section, the input light power target value of the second optical amplifying means is obtained on the basis of noise light power due to Raman amplification computed in accordance with the power of Raman amplification excitation light detected by the excitation light power detecting section, and on the basis of the noise characteristics of the second optical amplifying means. The excitation light controlling means executes its controlling operation in accordance with this target value.
As another aspect of the optical amplification apparatus described above, the target value setting means may set a maximum value of input dynamic range of the second optical amplifying means to the input light power target value of the second optical amplifying means. Further, in this case, it is preferred that when excitation light power of the first optical amplifying means reaches a maximum value before the input light power of the second optical amplifying means reaches the maximum value of the input dynamic range, the target value setting means sets the input light power target value of the second optical amplifying means so as to correspond to the maximum value of excitation light power of the first optical amplifying means.
According to this aspect, the input light power target value of the second amplifying means is set in the target setting means to the maximum value of the input dynamic range of the second optical amplifying means, and the excitation light controlling means executes its controlling operation in accordance with the target value. At this time, if the excitation light power for Raman amplification reaches the maximum output before the input light power of the second optical amplifying means reaches the maximum value of the input dynamic range, the input light power corresponding to the excitation light power at that point is set as the target value.
The optical amplification apparatus described above may further include input light power detecting means for detecting the input light power of the second optical amplifying means, and the excitation light controlling means may control the excitation light supply condition of the first optical amplifying means so that a detection result of the input light power detecting means coincides with the target value set by the target value setting means. This construction makes it possible to perform a feedback control that keeps the input light power of the second optical amplifying means to be constant at the target value.
As a specific construction of the optical amplification apparatus described above, when the second optical amplifying means includes a plurality of optical amplifying sections connected in parallel with one another, the excitation light controlling means may set an input light power target value corresponding to each of the optical amplifying sections. A specific construction of the second optical amplifying means may include an optical fiber amplifier using a fiber doped with a rare earth element.
With a method of controlling an optical amplification apparatus utilizing Raman amplification according to the present invention, in the optical amplification apparatus comprising: first optical amplifying means for Raman amplifying signal light propagated through a Raman amplification medium by supplying excitation light to the Raman amplification medium; and second optical amplifying means for amplifying the signal light output from the first optical amplifying means, a target value for minimizing a noise figure of the overall optical amplification apparatus as to input light power of the second amplifying means is set and an excitation light supply condition of the first optical amplifying means is controlled in accordance with the target value.